Optical Coherence Tomography (OCT) is an imaging technique whereby the interference properties of low-coherence light is used to generate real-time, cross-sectional and three-dimensional images. Analogous to ultra-sound, which measures reflecting sound waves, OCT measures the echoes of back scattered light passing through a sample to generate detailed sub-surface images.
OCT provides a means of imaging inhomogeneous samples, such as biological tissue. For example, due to the transparent ocular structures of the human eye, OCT technology can be used to detect macular holes, edema and degeneration as well as other eye disease. It can also be used for in-body imaging, via catheter or endoscope, to generate intra-vascular or intra-organ imaging scans.
OCT is an interferometric technique that splits and recombines an optical source to detect the differences between superimposed waves. Unlike ultrasound technology, which measures the time delay of a generated sound wave, interferometry measures the time delay and intensity of reflected light.
Interferometers generally consist of a single beam of light, split into two waves by a beam splitter, to generate a reference beam and sample beam of light with the same frequency and phase. In a Michelson-Morley interferometer, half transparent mirrors are used as beam splitters. Interferometry can provide a measurement of light waves reflecting off surfaces to produce an interferogram which depicts output amplitudes as a function of delay between the input spectra. The interferogram is subsequently analyzed, for example by Fourier transform analysis, to determine how the light has been altered through contact with the sample.
An OCT system generally consists of source, sample, reference and detector arms. The source light enters the interferometer and is then split into the sample arm and the reference arm. In many imaging systems, the phase of the reference beam is delayed by physically changing or adjusting the optical path distance of the reference arm which is reflected back towards the beam splitter. The sample arm reflects and/or disperses off surfaces of the sample being scanned to generate a return arm. When the two beams of light, are recombined with one another, the resulting interferogram can be recorded. The introduction of the delay allows for depth analysis of the sample, since the reference beam is delayed by a known path length equal to the depth of the sample at a particular point.
The field of OCT can be classified into two main categories of processing techniques: Time Domain OCT (TD-OCT) and Frequency Domain OCT (FD-OCT). In TD-OCT, the optical delay in the reference arm is rapidly varied, and the amount of light reflected at a specific depth of the sample can be calculated by measuring the strength of the interference signal as a function of time. Each depth corresponds to a different time step, which is measured using the reference arm. By matching the path lengths of the sample arm and reference arm of light, the back-reflected light can be constructively interfered with the light from the reference arm. Thus, by varying the optical path distance, or delay, different depths of the sample can be imaged. To generate 3D-images, OCT synthesizes cross-sectional images from a series of laterally adjacent depth-scans.
In FD-OCT, the depth information in the signal is extracted by measuring the interference spectrum of the signal. The delay in the reference arm is typically fixed, but the illumination source is broadband light, so different wavelengths of light will experience different amounts of interference, which can be measured by sending the broadband interference signal through a dispersive spectrograph. FD-OCT can increase imaging speed by allowing for imaging of all depths at once. Unlike with TD-OCT where rapid image acquisition can be limited by mechanical scanning in two directions (axial and lateral), FD-OCT techniques can tend to provide faster OCT imaging as they may require only one mechanical lateral scan.
Independent of the FD-OCT technique, spectroscopic analysis of light reflected from a sample can also be used to determine the composition and material structure of a sample. This technique is based on the principle that every molecular structure exhibits a unique absorption pattern, with absorption peaks corresponding to the frequencies of vibrations between the bonds of the atoms making up the material. Since the intensity of spectral features in reflectance is a function of the intrinsic absorption strength, scattering properties, and abundance of a material, no two compounds produce the same spectrum, and thus tending to allow for positive identification of materials.
In FD-OCT systems because a given feature in an observed spectrum can be due to either molecular absorption or interferometric signal, and the two phenomena cannot be distinguished a priori without additional information, spectroscopic molecular analysis cannot be combined. The current invention describes several approaches for simultaneously extracting spatial and spectral information from a sample by collecting additional information which removes the ambiguity between interference fringes and absorption features.
In OCT three-dimensional (3D) imaging, resolution can be defined in both the transverse and axial directions. The axial resolution is limited by the coherence length of the illumination source, which is inversely proportional to the spectral bandwidth. Because it uses low coherence light interference, conventional OCT systems can provide high resolution imaging data; however, the lateral resolution can be limited by an insufficient transversal sampling rate or the size of the probe beam diameter.